1. Field of the Invention
The present invention relates to the technical domain of thermal recovery of heavy oil, in particular to a method and an apparatus for improving the steam dryness of a steam injection boiler, which can reduce thermal loss, improve dryness of injected steam, increase steam enthalpy, replenish stratum energy, and improve oil recovery rate.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Presently, the heavy oil recovery method mainly used in China and foreign countries is thermal recovery, which injects thermal energy into the oil layer, so as to increase oil layer temperature, decrease oil viscosity, improves oil fluidity, and attain the purpose of development and production. Steam generators (DC steam injection boilers) are of a specialized appliance commonly used for heavy oil recovery in China and foreign countries at present, and there are more than 2000 steam generators in China. The steam dryness is a key index that has influence on the steam injection and development effect. The higher the steam dryness is, the higher the steam enthalpy will be, the higher the oil volume swept by the steam in the oil reservoir will be, and the better the development effect will be. At present, limited by the structure of steam injection boiler and the quality of supplied water, the steam dryness at the outlet of steam injection boiler is only 70%-75%, and the steam dryness will be only 30%-40% when the steam is injected to the bottom of the well, owing to the thermal loss in the wellbore, causing severely degraded thermal recovery effect. To improve steam dryness, a superheated steam injection boiler is designed, with the following working principle: the steam at the outlet of a steam injection boiler enters into a spherical steam-water separator arrange outside of the boiler body for steam-water separation, the separated high-dryness steam is returned to a superheat section of the steam injection boiler and is heated up into superheated steam, the superheated steam flows through a spray-type desuperheater arranged outside of the boiler body and is mixed with the saturated brine separated from the spherical steam-water separator, so that the saturated water is vaporized to improve the dryness, and then is injected to the bottom of the well. Though that method avoids salt precipitation in the heat affected area and improves steam dryness, but the equipment has a complex structure and requires many auxiliary devices, and the control is complicated and the production cost is very high; if such boilers are used to substitute the existing steam injection boilers, the existing equipment will be wasted, and the investment is huge.
In addition, thermal recovery of heavy oil mainly employs steam huff-puff and steam flooding, and belongs to a pressure depletion process; as the huff-puff cycles increase and the steam flooding time extends, the stratum energy will become severely deficient, the development effect will be degraded, and the recovery rate will be decreased. The steam injection boiler can't solve the problem of stratum energy deficit in itself.